The natural sense of hearing in human beings involves the use of hair cells in the cochlea that convert or transduce acoustic signals into auditory nerve impulses. Hearing loss, which may be due to many different causes, is generally of two types: conductive and sensorineural. Conductive hearing loss occurs when the normal mechanical pathways for sound to reach the hair cells in the cochlea are impeded. These sound pathways may be impeded, for example, by damage to the auditory ossicles. Conductive hearing loss may often be overcome through the use of conventional hearing aids that amplify sound so that acoustic signals can reach the hair cells within the cochlea. Some types of conductive hearing loss may also be treated by surgical procedures.
Sensorineural hearing loss, on the other hand, is caused by the absence or destruction of the hair cells in the cochlea, which are needed to transduce acoustic signals into auditory nerve impulses. People who suffer from sensorineural hearing loss may be unable to derive significant benefit from conventional hearing aid systems, no matter how loud the acoustic stimulus. This is because the mechanism for transducing sound energy into auditory nerve impulses has been damaged. Thus, in the absence of properly functioning hair cells, auditory nerve impulses cannot be generated directly from sounds.
To overcome sensorineural hearing loss, numerous cochlear implant systems—or cochlear prostheses—have been developed. Cochlear implant systems bypass the hair cells in the cochlea by presenting electrical stimulation directly to the auditory nerve fibers by way of one or more channels formed by an array of electrodes implanted in the cochlea. Direct stimulation of the auditory nerve fibers leads to the perception of sound in the brain and at least partial restoration of hearing function.
When a cochlear implant system is initially implanted in a patient, and during follow-up tests and checkups thereafter, it is usually necessary to “fit” the cochlear implant system to the patient. Fitting of a cochlear implant system to a patient is typically performed by an audiologist or the like who presents stimulation pulses to the patient and relies on verbal feedback from the patient as to how such stimulation pulses are perceived. Adjustments to the stimulation pulses may be made in response to the verbal feedback.
In addition to verbal feedback from the patient, the audiologist may look for non-verbal feedback provided by the patient, such as changes in facial expression, that may be indicative of a response of the patient to the applied stimulation pulses. Non-verbal feedback is especially important in pediatric fittings or in other situations in which verbal feedback is unavailable. However, when fitting a bilateral cochlear implant patient (i.e., a patient who has a separate cochlear implant for each ear), it is often difficult for the audiologist to know which cochlear implant elicits such non-verbal feedback.